How can you use the word diet?

[jdog]

I had a conversation with someone today and I would like to know what other people think. I thought it would be appropriate to use the word diet in the following sentence ,I drink Shasta for my diet. He then said it was I drink Shasta in my diet. Which is the correct way of saying it. I was thinking since diet can mean the food you eat or drink regularly I could say it for my diet. I have never said it before but now i just want to see what other people think. I do not mean diet as in weight loss.